While no single electrocardiographic abnormality is characteristic of a population of patients with hypertrophic cardiomypathy, two-dimensional echocardiography clarifies the significance of certain electrocardiographic patterns: (1) left ventricular hypertrophy on ECG, although present in only about one-half of the study population, was a relatively sensitive (i.e., 74%) marker for extensive (type III) left ventricular hypertrophy; (2) abnormal Q waves can not be explained by ventricular septal hypertrophy alone; (3) a normal electrocardiogram was most commonly a manifestation of localized left ventricular hypertrophy.